Rich the Kid's been arrested for possession of a concealed weapon at LAX ... law enforcement sources tell TMZ.

We're told the rapper was going through security at the airport Monday afternoon when a loaded firearm was discovered in his carry-on luggage. He was busted shortly after 1 PM PT, but it's unclear if cops have processed him yet.

The LAX arrest comes on the heels of a pretty rough 2020 that saw Rich get sued a few times -- by a management company, a jewelry store, and Fashion Nova.

The year before wasn't much better ... when he was the apparent target of a shooting attack at a West Hollywood studio -- and in 2018, he was badly injured in a home invasion.

The calendar change isn't helping Rich much ... not yet, anyway.